This is a 371 of PCT/EP 00/05050 filed Jun. 2, 2000 and the priority of DE 199 26 002.8 filed Jun. 8, 1999 is claimed.
The invention concerns a method for testing the integrity of filter units utilizing hydrophobic filter materials by a qualitative water intrusion test and apparatus for carrying out the method.
Such filter units comprise a housing, which is separated by a hydrophobic filter into a feed or inlet side chamber and a permeate or outlet (clean) side chamber. The inlet and outlet side chambers are equipped with fittings, so that the medium to be filtered (gas, hydrophobic liquid) is forced to pass from the inlet to the outlet. Undesirable substances, for example microorganisms, are retained by the filter material. Filter materials may include membranes or non-wovens from the areas of ultrafiltration and microfiltration.
Water intrusion filtration integrity tests on such hydrophobic filtration units are known. Such tests are non-destructive and utilize the characteristic of a hydrophobic filter to reject the entry of water into its pores. Notwithstanding the filter""s resistance to the permeation of water, if pressurized water is introduced, the water will then, proportional to the pressure, permeate into the pore structure of the filter. This nearly linear threshold pressure zone is known as an xe2x80x9cintrusion zone.xe2x80x9d If the water pressure is increased, a pressure threshold is reached, at which the permeation of water increases exponentially, known as the xe2x80x9cpenetration zone.xe2x80x9d This pressure threshold corresponding to the pressure at which the water breaks through the filter""s pores, is affected by the size of the pores of the hydrophobic filter material and may be used to characterize the filter""s largest pores. The intrusion zone and penetration zone values for various hydrophobic filter materials are available from filter manufacturers.
To carry out a water intrusion test, the test pressure for the filter material is adjusted to the intrusion zone value and maintained for a specified time. Then, by measuring the pressure drop following said specified time, the amount of water intruded into the pore structure of the filter is measured in terms of per time units. From this intrusion rate measurement conclusions can be drawn in regard to the degree of integrity of the filter material; in the case of intact filter material, the rate of intrusion is very small, while in the case of damaged material, it is correspondingly larger.
In DE 39 17 856 C2 a testing device and a procedure for carrying out an in-line water intrusion test is disclosed. The testing device operates pneumatically and comprises a filter housing for the filter units to be tested as well as the required connection lines and valving. The filter housing, which is fitted with the test filter units, is completely filled with water on the inlet chamber and subsequently subjected to compressed air at a test pressure of 70 to 80% of the penetration zone pressure value that is typical for the filter elements of the test filter units. After a stabilization phase with closed valves, the pressure drop in the system is measured by an upper connection on the filter housing. The disadvantage of this method is that the procedure is too expensive and time-consuming for testing a large number of filter units because each filter unit must be inserted and removed from the filter housing.
In DE 43 39 589 C1 there is also described a quantitative procedure and a test device, which starts with the calculation of those volumes which relate to each test pressure, as compared to a total net volume of the test equipment (determined for a given testing device as a testing device constant).
To avoid large dead volume caused by elongated pressure gas lines, which can lead to errors in pressure readings with variations in temperature, DE 195 03 311 A1 proposes the incorporation of an external electrical pressure sensor at the connection of the filter housing. To monitor and record measurements, this sensor is connected electronically to an evaluation unit. The disadvantage here is that for different types of filter units, either the particular entire net volume must be determined or an external reference container must be employed, which is time-consuming and adds to the complexity and expense of the testing. A further disadvantage is that usable integrity test results can only be obtained when the surface area of the filter unit to be tested is relatively large, because otherwise the pressure drop caused by the water intruding into the filter""s pore structure cannot be determined with the necessary precision. Experience has demonstrated that the surface area of filter units to be so tested should be greater than 1000 cm2.
Thus, the present invention has the goal of providing a method for testing the integrity of hydrophobic filter units by a water intrusion test, whereby filter units having smaller surface areas can be simply and routinely tested in large numbers, and further, of providing test apparatus for carrying out said method.
In accordance with the invention, a simple method for testing the integrity of hydrophobic filter units is provided to carry out a qualitative water intrusion test to exclude unusable filter units. A significant advantage to the method is that unusable filter units with serious defects are detected immediately at the start of the test, so there is often no need to continue the test protocol to its end. Another advantage of the method is that it is not limited to tests of filtration units having large surface area filters.
The invention is applicable to testing the integrity of filter units used in the aeration of laboratory fermentations, sterilizers, lyophilizers, sterile tanks and small containers such as glass flasks or for the filtration of hydrophobic liquids.